ELECTRIC DRIVE TRAIN TEST – QUO VADIS?The development of electric drive trains generates new tasks regarding the test methods of individual vehicle

components as well as the verification process of the drive train as a whole. The experiences gained over the

years from test specifications and test cycles of the conventional drive train are not immediately transferable.

Added to that, there are also newly-defined processes within the producing companies. The portfolio specifically

designed for this purpose by Scienlab electronic systems provides an open interface with different test bench

control systems, which in turn enables an incremental modular concept for a target-oriented and effective test

method of electric drive trains.

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CONTINUOUS TEST ENVIRONMENT

Over the past decades, the developmentprocess of the conventional drive train, ranging from a model-based develop-ment procedure to test benches, whichare used to verify entire vehicle drivetrains, has been optimized and perfectedaccording to changing conditions. How-ever, the electric drive train differs con-siderably from a conventional one. Thus,experiences made so far are not easily oronly partially transferable.

Energy storage has developed from asimple battery with two poles to a bat-tery system with control units andthermal management. At the same timethe energy content of the storage hasincreased tremendously. Accordingly,voltages of up to 1000 V are currently measured in test benches and wattagesof several 100 kW with currents close to 900 A are transferred. The inverter,which is connected to the vehicle’s high-voltage on-board power supply andwhich regulates the electric machine, isan entirely new component in a vehicleof this performance class. Previously unknown engine speeds and torquecharacteristics also occur at the mechan-ical output. In addition, there are furtherhigh-voltage components such as charg-

ers, DC/DC converters, air conditioning, heating and other auxiliaries with a highenergy demand. The basic structure of an electric drive train is illustrated in ❶.

This modified structure not only affects testing methods, but also well-established structures within companies, which until recently had developed con-ventional drive trains. An example for this concerns the energy management, which would typically be found in a 14 V vehicle electric system and whichis now considered as an essential chal-lenging task regarding electric drive trains.

In order to achieve this goal it seemsreasonable to classify the test task and define the test environment accordingly. By means of these classes this article introduces a continuous test environ-ment for an electric drive train as well as involved development and production processes, ranging from HiL environ-ments for individual components to the combination of all components. In this context we shall present a system, which enables development and test proce-dures by means of a gradual concept. A particularly interesting alternative con-cerns the demarcation of “low-energy”test benches and test benches with full electric functionality.

DR.-ING. ROGER UHLENBROCKis Director of Scienlab

electronic systems GmbH in Bochum (Germany).

DR. MICHAEL SCHUGTis Director of Scienlab

electronic systems GmbH in Bochum (Germany).

DIPL.-ING. THOMAS SPECKBROCKis Head of Marketing/Sales

at Scienlab electronic systems GmbH in Bochum (Germany).

AUTHORS

❶ Basic structure of an electric drive train

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CLASSIFICATION OF TEST METHODS WITHIN THE ELECTRIC DRIVE TRAIN

In principal, one component in an electric drive train consists of a control unit and an electric function unit. The battery is monitored and controlled by a battery man agement system (BMS), the inverter by an according motor control. The electric drive train then consists of the combina-tion of these components (energy storage – in verter – electric machine) with a superordi nate vehicle control unit. With the objective to be able to test and verify each com ponent independently in the development process as early as possible, a test methodology can be classified into the following three classes.

The first class deals with the testing process of control units at signal level. Here, the power-electronic part is emu-lated. This means that all necessary input and output signals are simulated in a HiL environment. In addition to affect-ing the communication and sensor sig-nals, this also affects the power-elec-tronic interface. For example, individual cell voltages of a battery can be emulated by means of high-accuracy voltage sources. A major advantage is the elimi-nation or at least a reduction of high-energy dangers. This class is therefore specifically suited for the development of software algorithms for control units. Dif-ferent test cases can be easily generated by parametrization and each test can be accurately reproduced.

The second class refers to functional testing procedures of individual compo-nents, including the control unit and bidirectional energy flow. This addresses the model-based power-electronic inputs and outputs of the components by means of emulators. In case of an inverter, for in stance, this involves the emulation of a battery on the DC side and the emulation of an electric machine on the AC side. A test bench control allows the selection of these systems and the control unit of the inverter. This test method bears the essential advantage that each component can be fully and independently tested at a very early stage of the development process. Due to software parameters the marginal conditions, including the load on the test item, are arbitrary, from quasi-static loads to highly dynamic

processes. Examples include the simula-tion of start and brake processes or any other random driving cycles. The flexible alteration of test conditions and the pos-sibility of component-independent tests result in substantial time savings and, accordingly, in cost advantages.

The third class of test methods com-plies mostly with the conventional meth-odology, where certain parts or all com-ponents are combined and tested. Here, different drive concepts (2WD/4WD) can be simulated on the mechanical side. During this test, energy storage is usually emulated with DC voltage sources in simpler cases and battery emulators in more sophisticated tests.

TESTS AT SIGNAL LEVEL

Due to electric dangers, based on high voltages in combination with possible high currents, a test at signal level pro-vides ma jor advantages regarding the development of software algorithms of control units. In addition to the repro-ducibility of the tests and simultaneous flexible parametrization, the prevention of high energies within the test system should be taken into account.

A good example for this is provided by the HiL test environment for battery management systems (BMS). In addition to various current, electricity and tem-perature signals, control signals for con-tactors and, if necessary, pumps of the thermal management, it is the cell volt-ages, which are particularly needed in order to emulate all the necessary inputs and outputs. Instead of using real cells with a very high energy content it is more advantageous to emulate these as voltage sources. Needless to say, care must be taken to ensure that high-accu-racy and dynamics suit the test item. In order to realize a low discharging curve of a specific cell chemistry, the output voltage of the individual channels must be precisely adjustable, else the BMS is unable to perform a SOC determination.

The maximum current provided by the voltage sources should be kept very low in order to keep the energy within the test system as low as possible. In case the so-called load-balancing should also be emulated, the sources must be capa-ble of providing the according current, which softens the energy poverty of the

test system to a certain extent. However, software algorithms of the BMS can still be developed, tested and verified under very comfortable emulating conditions.

TESTS AT A PURELY ELECTRICAL LEVEL

Regarding the development of new elec-tric drive trains it is particularly desirable to be able to test the individual electric components independently. A paralleliza-tion of development processes therefore contains substantial advantages with regard to time and money. The require-ment for a test at a purely electrical level is an apt emulation of the power-elec-tronic interface.

In terms of the energy storage this inter-face is obviously represented by the high-voltage on-board power supply. This can be realized by means of suitable charge/discharge processes and with the help of accurate and reliable energy storage test systems. Regarding a battery system, aspects such as the inclusion of the BMS and the simultaneous control of environ-ment temperature and the conditioning of cooling water must also be synchronized during the test procedure. Due to the high energy content within the test item it is necessary to allow for suitable safety fea-tures [1, 2].

Conditions prove to be more elaborate regarding the inverter. On the one hand, an energy storage provides the necessary energy on the DC voltage side, and further aggregates are connected to the high-volt-age on-board power supply as consumers on the other. A simplified emulation with a DC voltage source is often inexpedient, as dynamic processes cannot be emulated accurately enough. This can be solved by battery emulators with a fast interface with the test bench control, which are also cap able of emulating short-time cur-rent gradi ents on a sub-millisecond times-cale. These gradients occur during rapid changes between generator and motor operation.

An electric machine with a driveshaft and braking motor would usually be connected on the AC side for test pur-poses. A far more flexible solution on a mere electrical level can be achieved with a machine emulator. In this case a ma -chine model determines the three phase currents as a reaction to the inverter’s

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demand. The in verter receives further information from the model regarding therotor position and temperature. A furthercommunication between the components does not take place, which is an equiva-lent behavior to the real electric machine. When modeling this kind of machine, the perception limits of the test item shouldbe taken into account. A machine modelwith characteristics, which cannot be really perceived by the inverter, is inade-quate. Another advantage involves the flexible parametrization of machine char-acteristics up to and including differentmachine types. This enables a quick oper-ation and thus a reduction of develop-ment periods.

The control of the test sequence and the vehicle simulation take place in the superordinate automation system. Inorder to guarantee the real-time capabil-ity of the test bench, it is necessary toensure a fast communication connection

between the test bench and the emula-tors. A concept that proved suitable comes in form of the Dual-Ported-RAM, where both systems access a common memory via high-speed serial connections. While, in terms of the energy storage, it is still possible to calculate the model on the automation system, this process is limited in terms of the machine. Dynamic pro-cesses require cycle rates on a micro-sec-ond scale, which cannot be realized via communication lines.

❷ illustrates the basic test environ-ment for an inverter. In addition to the already mentioned emulators further emulators are needed, such as mains emulators for chargers and low-voltage emulators for the simulation of the vehi-cle’s 14 V power supply network. Fur-thermore, a test bench guard connected to the facility management is neededwho watches over the installation safety. Depending on demands, an environment

simulation with a temperature chamberand cooling-water-conditioner may alsobe required.

Other vehicle concepts can also betested in this manner. It is possible, forinstance, to test power electronics of afuel cell vehicle in such an emulated power-electronic environment. In this case, the specific characteristics of the fuelcell are calculated and allocated to the testitem by means of an appropriate power-electronical device. An exemplary testenvironment is illustrated in ❸. Different test environments, specifically arranged for the items under test, can be imple-mented by applying a modular systemconcept. Common intermediate circuits of emulators, in which the energy is circu-lated, are also possible. Thus, only the losses of the test environment are taken from the supply network. In addition to cost savings regarding the investment, this structure also reduces the costs of

❷ Test environment for an inverter of an electric drive train

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implementing the laboratory media as well as the costs of the subsequent commissioning.

ELECTROMECHANICAL TESTS

In order to test and verify the electricdrive train as a whole, all the individualcomponents are operated in one testbench. At first sight, the test involving the inverter and the electric machine, in par-ticular, seems to present a classic test bench. Howwever, it also implies new chal-lenges. Here, the focus lies on the signifi-cantly higher speed and altered course of torque as compared to the classic drivemechanisms. Accordingly, this requires new setups and braking machines. A test bench such as this one can also be employed to analyze different drive con-cepts such as two-wheel or all-wheel drives, or even hybrid approaches such asrange extender etc.

In addition to the mechanical loadthese test benches often don’t use the original energy storage, due to the lim-ited energy and due to the poor repro-ducibility of test conditions. Emulators,which can be parametrized prove signifi-cantly more flexible. These are imple-mented according to the above men-tioned concept.

SUMMARY

The electric drive train requires a change of view regarding the test methods of itscomponents in comparison to theconventional drive train. Early tests of control devices at signal level, followed byindependent, mere electrical tests of eachcomponent and, finally, overall electrome-chanical tests are in demand. This leadsto reduced setup and development times.Costs for developing electric drive trainscan hereby be clearly reduced. A modular

concept for the implementation of these test benches en ables a flexible reaction to power-electronic demands with a suitable test methodology.

REFERENCES[1] Doerlemann, Ch.; Muss, P.; Schugt, M.; Uhlenbrock R.: Sichere und flexibleTestumgebung für Lithium-Ionen Batterien. In: ATZelektronik 05|2008, pp. 50 – 55[2] Kern, R.; Bindel, R.; Uhlenbrock, R.; Durch-gaengiges Sicherheitskonzept für die Prüfung von Lithium-Ionen-Batteriesystemen. In: ATZelektronik05|2009, pp. 22 – 29

❸ Test environment for power electronicswith common intermediate circuit

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